A positron emission mammography (PEM) instrument is being developed for use as a clinical and research tool for the diagnosis and treatment planning of breast cancer. Dedicated instrumentation for PEM was implemented in a standard X-ray mammography unit by positioning two gamma cameras, one above and one below a compressed breast. The radiotracer fluorodeoxyglucose (FDG) is then administered to the patient. FDG is known to concentrate in breast tumors, thus making it detectable in positron emission tomography (PET). Images of the distribution are obtained in real time and displayed. The development process includes the design and fabrication of the PEM unit itself, as well as electrical and mechanical instrumentation required for calibration and acceptance testing. Preliminary clinical trials were performed during this period.